I have two meer-lined pipes. Both smoke well, I treat them like meerschaums as far as cleaning (I carefully scrape out any tobacco/ashes left in the bowl. use a paper towel to "ream out" the interior of the bowl after each smoke and do not let any cake form) and am careful not to bang 'em around -- they are briar, but meerschaum is delicate and will crack and/or break rather easily.
The meerschaum in some meer-lined briars is "pressed meerschaum," not "block." Which is cheaper and probably more susceptible to damage. How/why this happens is likely due to being dropped, banged, etc., which often won't be noticed, but when the resultant crack in the meer lining occurs, it's really just a matter of time before, eventually, it breaks down further and pieces start falling out.
The good news is, you likely can still smoke the pipe -- just remove all the meerschaum lining. It's a briar pipe. You will have some "break-in" to do, but it will probably smoke well.
With meerschaum-lined pipes, as you know now, they have to be treated gently. Also, you get what you pay for -- many, if not most meer-lined pipes are indeed pressed meerschaum and are priced in the lower mid-range. Some of the better ones of those are French make.
I hope this answers your questions